1. Field of the Invention
This invention relates to improvements in the thermal stability and flowability, in the molten state, of saponified ethylene-vinyl acetate copolymers.
2. Description of the Prior Art
Saponfied ethylene-vinyl acetate copolymers are generally produced by saponifying ethylene-vinyl acetate copolymers with a caustic alkali or an alkali metal alcoholate. When the saponification products are subjected to melt molding or melt film-forming, they are susceptible to thermal degradation, whereby the melt viscosity decreases and discoloration takes place to an extent preventive of practical use thereof. For the saponification products to be practicable, improvement both in thermal stability and in moldability is essential.
So far a number of measures for improving these properties have been proposed. For example, Japanese Patent Publication No. 37664/1971 and Japanese patent application Kokai (laid open) Nos. 25048/1973, 88544/1976 and 88545/1976 disclose that adequate washing of the resin with water, addition of an acid thereto or immersion thereof in an acid solution can prevent heat degradation or heat discoloration to some extent. Japanese Patent Publication No. 5781/1973 and Japanese patent application Kokai No. 954/1977 disclose the addition of metal salts of a certain kind, which are claimed to have a remarkable stabilizing effect, for improving thermal stability of the resins. Furthermore, Japanese patent application Kokai No. 956/1977 discloses that coexistence of such an acid and such a salt can produce a favorable synergistic effect and at the same time stabilize the hydrogen ion concentration of the treating solution.
However, these prior art methods, though effective to a certain extent, cannot overcome the dilemma that satisfactory improvement in thermal stability can be achieved only at the cost of flowability in the molten state; whereas, adequate improvement in flowability can be attained only at the cost of thermal stability. Decreased flowability may result in roughened film surface and, in extreme cases, fish eye formation or film break, and insufficient thermal stability may lead to remarkable discoloration during melt molding.
Furthermore, the most fatal disadvantage in the prior art is that heating in an oxidizing atmosphere, such as air, for a relatively long period of time not only causes saponified ethylene-vinyl acetate copolymers to become unmeltable, whereby the copolymers cannot remain in the molten state even with continued heating, but also causes discoloration to a yellow to yellowish brown color. Even if the saponified copolymers can be melted with heating, the melting is very often incomplete and the surface of the melted resin bears a yellow to yellowish brown color. Since saponified ethylene-vinyl acetate copolymers are thermally stable under acidic conditions, most of the prior art methods of stabilization against heat consist in maintaining the saponified copolymers within an acidic range. However, the higher the acidity of the saponified copolymer is, the more significant the above-mentioned phenomena of becoming unmeltable and discolored upon heating in an oxidizing atmosphere is. The prior art techniques thus cannot solve such a dilemma, hence the prior art techniques cannot provide saponified ethylene-vinyl actetate copolymers having good melt moldability.
A need therefore continues to exist for saponified ethylene-vinyl acetate copolymer compositions having a high degree of thermal stability and a good flowability in the molten state, hence a good melt moldability, and a method of making the same.